Heltec v4: Add cli command to toggle GPIO

[mattzzw]

This PR adds the capability to control a gpio pin defined in the target's platformio.ini by -D PIN_GPIO via command line.

gpio 1 will set the pin HIGH
gpio 0 will set the pin LOW

States are not persisted, rebooting resets the pin to LOW.
This could be useful to control other hardware next to the repeater, like e.g. starting a RPi next to the repeater.
Implementation should be straightforward for other targets.

[IoTThinks]

To turn on/off a fan in repeater?

Then why not "set gpio 1 on".
We can use for all available GPIOs.

So we do not need to set -D GPIO=1

[mattzzw]

To turn on/off a fan in repeater?

As I wrote above:
This could be useful to control other hardware next to the repeater, like e.g. starting a RPi next to the repeater.

Then why not "set gpio 1 on".

This PR expects one GPIO to be defined. There's probably a thousand ways to implement this.
I needed to be able to control one GPIO. Expanding this to multiple GPIOs for multiple targets was a bit beyond my scope and capabilities ;-)

[JDat]

What if I want to control 5 GPIOs? Code and config must be more generic.

[mattzzw]

Feel free to expand on this PR. Should be more or less straight forward, depending on the availability of GPIOs on different targets.

[JDat]

Can you make it already with multiple GPIO in one pull request, so no need to expand or submit code in future?